WASHINGTON — President Obama won bipartisan backing Tuesday for strikes on Syria but final approval for the effort was far from certain, with Secretary of State John F. Kerry facing tough questions during his Senate testimony about whether US ground troops could eventually be deployed.

The good news for Obama came early in the day when House Speaker John Boehner and House minority leader Nancy Pelosi emerged from a White House meeting to announce they would support limited strikes against Syria. The House’s second-ranking Republican, Eric Cantor of Virginia, also released a statement supporting the president.

“This is something that the United States as a country needs to do,” Boehner said. “I’m going to support the president’s call for action; I believe my colleagues should support this call for action.”

But later in the day, Kerry, acting in his role as the administration’s point person, ran into a stream of skeptical questions when asked whether there was any condition under which Obama’s pledge against putting “boots on the ground” might be broken.

Kerry initially said that there might be circumstances under which US troops could enter Syria, such as if Syria “imploded” or if chemical weapons were transferred to other dangerous parties. “I don’t want to take off the table an option that might or might not be on the table,” Kerry said.

JIM LO SCALZO/EPA

Secretary of State John F. Kerry and Secretary of Defense Chuck Hagel conferred.

Senator Bob Corker of Tennessee, the top Republican on the committee, objected.

“I hope we work through something that’s much clearer,” Corker said. “I don’t think there are any of us here that are willing to support the possibility of having combat boots on the ground.”

Kerry quickly backtracked, insisting those were merely hypotheticals and would not be allowed under any bill Congress would consider in the coming days to make limited strikes.

“Let’s shut that door now as tight as we can,” Kerry said. “There will not be American troops on the ground with respect to the civil war.”

On Tuesday night, senators circulated a new compromise resolution that would authorize force, but only for up to 60 days, with one 30-day extension. The resolution prohibits the use of ground forces and is far more limited than the one that Obama had submitted to Congress on Saturday. The White House has said it would be willing to compromise.

As the decision for Congress drew closer, most lawmakers were openly grappling with what is emerging as one of the most important foreign policy debates of the past decade. Every senator on the Committee on Foreign Relations returned early from recess to attend Tuesday’s hearing, and most said they would also be back on Wednesday for a classified briefing.

“This debate is about the world’s red line — it’s about humanity’s red line,” Kerry said to open the hearing. “And it’s a line that anyone with a conscience ought to draw.”

There was a sense of historic symmetry to the day. In 1971, after Kerry had returned from the Vietnam War and became a leading protester, he appeared before the same panel, the Senate Foreign Relations Committee, to urge an end to that conflict. On Tuesday, Kerry was the one being protested by some vocal members of the public, some of whom were ushered out of the proceeding.

In his role as secretary of state, the former senator sought to explain to his one-time colleagues the fine line that the administration is walking: while the White House would like to see Bashar al-Assad removed from power in Syria, Kerry stressed that the military strikes are not intended to change the balance of the civil war there or draw the US military into a ground campaign.

Senators asked why the United States had not acted sooner in Syria, what the administration was doing to ensure Assad would not use a US attack to brag that he had faced down the powerful nation, and how much international support the administration had.

Kerry said the level of international support would depend on the specifics of the military action, which Obama has yet to define. In any case, he said the military has the support it needs.

At another point, Kerry conceded that Assad would survive the attacks, and probably brag about doing so, but he insisted that the Syrian leader would be weakened militarily.

“He will be able to stand up and no doubt he will try to claim that this is something positive for him,” Kerry said. “There is no way that it will in fact be beneficial for him.”

In the lead-up to votes, expected next week, the White House has been engaged in an intense lobbying campaign, offering classified briefings and explaining Obama’s rationale for wanting to punish the Syrian regime for allegedly using chemical weapons on its own population.

Obama, who summoned top House and Senate lawmakers to the White House on Tuesday morning, opened the meeting by emphasizing that his plan would be limited and would involve air strikes — not ground troops.

“This is not Iraq and this is not Afghanistan,” the president said. “This is a limited, proportional step that will send a clear message not only to the Assad regime, but also to other countries that may be interested in testing some of these international norms, that there are consequences.”

While gaining support from Boehner and Pelosi provided momentum, it does not make the vote a certainty, particularly in a volatile House of Representatives, where even routine matters no longer pass. Boehner and Pelosi also plan to make the topic a so-called vote of conscience, meaning they will not push their members to vote one particular way.

Among those who are unconvinced are some Tea Party-backed Republicans, who tend to have a more isolationist view of foreign policy and want to keep the United States out of foreign conflicts, and some liberal Democrats who are reluctant to use military force until all other options have failed.

Many in the all-Democratic Massachusetts delegation said they remain skeptical of the plan. Representative Niki Tsongas, a Lowell Democrat and a member of the Armed Services Committee, said her office has received many calls from constituents opposed to military action. She said she has yet to make up her mind as she prepares for a classified briefing on Thursday.

Senator Edward Markey, the Massachusetts Democrat who succeeded Kerry and now serves on the Foreign Relations Committee, provided few clues about which way he was leaning. During his time for questions, Markey asked Kerry to help declassify more information to help the public learn more about the situation. Kerry responded that the administration had already taken unprecedented steps to declassify information but did not want to release details that would reveal intelligence sources.

The White House organized conference calls and classified briefings over the holiday weekend, and that was followed by what has become an unusual sight in Washington: bipartisan endorsement from the Republican and Democratic leaders of the House, Boehner and Pelosi.

“The United States for our entire history has stood up for democracy and freedom around the world,” Boehner said. “These weapons have to be responded to. Only the United States has the capacity and the capability to stop Assad or warn others around the world that this type of behavior will not be tolerated. I appreciate the president reaching out to me and my colleagues in Congress over the past few weeks.”

Pelosi said that while she thinks the American people need to hear more about the intelligence that supports the action, she believes Congress will pass the authorization. “We must respond,” she said.